Female reproductive biology of the silverside Odontesthes regia

Histological observations of gonads, size frequency distribution of oocytes, seasonal changes in the gonadosomatic index and fecundity were used to assess the reproductive biology of females of the silverside Odontesthes regia from October 2006 to November 2007 in southern Chile. Six developmental stages were characterised as a function of macroscopic and histological diagnostics of the ovarian tissue: (1) virgin and/or resting; (2) maturing; (3) late maturing; (4) hydrated; (5) spent and/or partially spent; and (6) recovering ovaries. Analysis of the size frequency distribution of oocytes of late maturing ovaries showed a distinctive gap between a heterogeneous group of pre-vitellogenic oocytes at the cortical alveoli stage (range = 0.05–0.6 mm) and a particularly homogeneous and advanced group of vitellogenic oocytes (range = 0.9–1.3 mm). Hydrated ovaries exhibited two distinctive features: (i) oocytes ranging from 1.8 to 2.5 mm and characterised by the presence of large chorionic filaments adhered to one extreme of the oocytes, giving them the appearance of a grape cluster, and (ii) a two-phase distribution of oocytes inside the ovary (i.e. hydrated and vitellogenic oocytes occupying the ventral region and dorsal region of the ovary, respectively). Post-ovulatory follicles were present in both recently spent and maturing ovaries. Irrespective of fish size, the mean IG showed that the spawning season extended mainly from August to December, where the IG varied from 3 to 30% in months with high reproductive activity. Batch and relative fecundity, which were assessed by counting and measuring hydrated oocytes, varied from 150 to 6000 oocytes and from 8 to 122 oocytes g−1, respectively. Overall results suggest that O. regia is a multiple-spawning fish with asynchronous ovarian development with indeterminate fecundity. The two-phase distribution of oocytes in hydrated ovaries seems to be a strategy to maximize spawning when chorionic filaments are adhered to large ovulated eggs.

► Silverside O. regia is a batch-spawner with indeterminate fecundity.
► This species produce large eggs and low batch fecundities.
► Hydrated oocytes have large chorionic filaments.